Re: What does IOSQE_IO_[HARD]LINK actually mean?

[Pavel Begunkov <asml.silence@gmail.com>]

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On 01/02/2020 15:02, Andres Freund wrote:
>> Right, after a "failure" occurred for a IOSQE_IO_LINK request, all subsequent
>> requests in the link won't be executed, but completed with -ECANCELED. However,
>> if IOSQE_IO_HARDLINK set for the request, it won't sever/break the link and will
>> continue to the next one.
> 
> I think something along those lines should be added to the manpage... I
> think severing the link isn't really a good description, because it's
> not like it's separating off the tail to be independent, or such. If
> anything it's the opposite.
> 
> 
>>> Looks like it's defined in a somewhat adhoc manner. For file read/write
>>> subsequent requests are failed if they are a short read/write. But
>>> e.g. for sendmsg that looks not to be the case.
>>>
>>
>> As you said, it's defined rather sporadically. We should unify for it to make
>> sense. I'd prefer to follow the read/write pattern.
> 
> I think one problem with that is that it's not necessarily useful to
> insist on the length being the maximum allowed length. E.g. for a
> recvmsg you'd likely want to not fail the request if you read less than
> what you provided for, because that's just a normal occurance. It could
> e.g. be useful to just start the next recv (with a different buffer)
> immediately> I'm not even sure it's generally sensible for read either, as that
> doesn't work well for EOF, non-file FDs, ... Perhaps there's just no
> good solution though.

People already asked about such stuff, you can find the discussion somewhere in
github issues for liburing. In short, there are a lot of different patterns, and
that's not viable to implement them in the kernel. There are thoughts, ideas and
plans around using BPF to deal with that.
I've sent LSF/MM/BPF topic proposal exactly about that.

> 
> 
>>> Perhaps it'd make sense to reject use of IOSQE_IO_LINK outside ops where
>>> it's meaningful?
>>
>> If we disregard it for either length-based operations or the rest ones (or
>> whatever combination), the feature won't be flexible enough to be useful,
>> but in combination it allows to remove much of context switches.
> 
> I really don't want to make it less useful ;) - In fact I'm pretty
> excited about having it. I haven't yet implemented / benchmarked that,
> but I think for databases it is likely to be very good to achieve low
> but consistent IO queue depths for background tasks like checkpointing,
> readahead, writeback etc, while still having a low context switch
> rates. Without something like IOSQE_IO_LINK it's considerably harder to
> have continuous IO that doesn't impact higher priority IO like journal
> flushes.
> 
> Andres Freund
> 

-- 
Pavel Begunkov


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